Are you still nursing?
It's her turn, you'll have to wait in line!
That. is. awesome.
I'd like to use it but not sure I'll have the guts...
But my go-to response is the puzzled stare, the one where you look at the questioner as if she has three heads. "Yeah...so? Don't you ever pick your kids up or cuddle them when they need comfort? That's just for security--but without the nutritional and immunological side benefits that breastfeeding has."
I've memorized a few statistics so well that I can rattle them off now even when I'm shocked and taken off guard. My favorites:
* The WHO recommends nursing till a minumum of age two.
* The average age of weaning worldwide is between the ages of two and four.
* The biological age of weaning is between the ages of five and seven.
And OP, if I were you, I would bring it up again and gently try to educate your SIL a little. At the very least to educate her about the fact that you want to keep your relationship with her positive and that her comment bothered you, and you'd appreciate it if she would try to avoid statements that undermine your parenting decisions--just as you try to avoid judging or making comments about HER parenting decisions. You are both moms, and you both know what is best for YOUR family. You are both following your instincts and making your own decisions, and neither of you has any call to do or say anything that undermines the other.